Gemstones Generally
There are a limited number of elements and chemical compounds that have the physical characteristics to be useful as gemstones. The physical characteristics that are generally accepted as being most important are hardness, refractive index and color, although thermal stability, chemical stability and toughness are also considered important in many gemstone applications.
To date, the only chemical substances technically considered precious stones are diamonds (single crystalline carbon) and corundum (sapphire and ruby [single crystalline aluminum oxide]) because their hardness when measured on the Mohs scale is approximately 9 or higher. The Mohs system is a scale for ranking hardness of a mineral with diamond being the hardest at 10, sapphire at 9, topaz , 8, down to the softest mineral, talc, which is 1. Emerald, because it is rare, is accepted as a precious stone even though its hardness is 7.5, while other gems, such as chrysoberyl, topaz and garnet, are usually classified as semiprecious stones because of their lower hardness. Hardness has practical value in that it defines the ability of a gemstone to resist scratching.
Refractive index is important because it defines the ability of a gemstone to refract light. When materials with a high refractive index are fashioned into finished gemstones they sparkle and appear brilliant when exposed to light. The characteristic sparkle of a diamond is due mainly to its high refractive index.
The color of a gemstone is determined by a variety of factors, from the impurity atoms that are available to be incorporated into the crystal lattice to the physical and electronic structure of the crystal itself. A ruby, for instance, is simply a sapphire single crystal (aluminum oxide) that contains a small concentration of chromium impurity atoms.
The thermal and chemical stability of a gemstone can be important during the process of mounting stones into jewelry. In general, it is beneficial if stones can be heated to high temperatures without changing color or reacting with ambient gases (that mar the surface finish).
The toughness of a gemstone relates to the ability of the gemstone to absorb energy without breaking, chipping or cracking. A gemstone must be able to withstand those impact forces normally encountered during a lifetime of use mounted on a ring or other jewelry item.
Hardness, refractive index, color, thermal/chemical stability and toughness are all characteristics that, in combination, determine the usefulness of a material as a gemstone.